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rfc:rfc903

Network Working Group Finlayson, Mann, Mogul, Theimer Request for Comments: 903 Stanford University

                                                             June 1984
               A Reverse Address Resolution Protocol
    Ross Finlayson, Timothy Mann, Jeffrey Mogul, Marvin Theimer
                    Computer Science Department
                        Stanford University
                             June 1984

Status of this Memo

 This RFC suggests a method for workstations to dynamically find their
 protocol address (e.g., their Internet Address), when they know only
 their hardware address (e.g., their attached physical network
 address).
 This RFC specifies a proposed protocol for the ARPA Internet
 community, and requests discussion and suggestions for improvements.

I. Introduction

 Network hosts such as diskless workstations frequently do not know
 their protocol addresses when booted; they often know only their
 hardware interface addresses.  To communicate using higher-level
 protocols like IP, they must discover their protocol address from
 some external source.  Our problem is that there is no standard
 mechanism for doing so.
 Plummer's "Address Resolution Protocol" (ARP) [1] is designed to
 solve a complementary problem, resolving a host's hardware address
 given its protocol address.  This RFC proposes a "Reverse Address
 Resolution Protocol" (RARP).  As with ARP, we assume a broadcast
 medium, such as Ethernet.

II. Design Considerations

 The following considerations guided our design of the RARP protocol.
 A. ARP and RARP are different operations.  ARP assumes that every
 host knows the mapping between its own hardware address and protocol
 address(es).  Information gathered about other hosts is accumulated
 in a small cache.  All hosts are equal in status; there is no
 distinction between clients and servers.
 On the other hand, RARP requires one or more server hosts to maintain
 a database of mappings from hardware address to protocol address and
 respond to requests from client hosts.

Finlayson, Mann, Mogul, Theimer [Page 1]

RFC 903 June 1984

 B. As mentioned, RARP requires that server hosts maintain large
 databases. It is undesirable and in some cases impossible to maintain
 such a database in the kernel of a host's operating system.  Thus,
 most implementations will require some form of interaction with a
 program outside the kernel.
 C. Ease of implementation and minimal impact on existing host
 software are important.  It would be a mistake to design a protocol
 that required modifications to every host's software, whether or not
 it intended to participate.
 D. It is desirable to allow for the possibility of sharing code with
 existing software, to minimize overhead and development costs.

III. The Proposed Protocol

 We propose that RARP be specified as a separate protocol at the
 data-link level.  For example, if the medium used is Ethernet, then
 RARP packets will have an Ethertype (still to be assigned) different
 from that of ARP.  This recognizes that ARP and RARP are two
 fundamentally different operations, not supported equally by all
 hosts.  The impact on existing systems is minimized; existing ARP
 servers will not be confused by RARP packets. It makes RARP a general
 facility that can be used for mapping hardware addresses to any
 higher level protocol address.
 This approach provides the simplest implementation for RARP client
 hosts, but also provides the most difficulties for RARP server hosts.
 However, these difficulties should not be insurmountable, as is shown
 in Appendix A, where we sketch two possible implementations for
 4.2BSD Unix.
 RARP uses the same packet format that is used by ARP, namely:
    ar$hrd (hardware address space) -  16 bits
    ar$pro (protocol address space) -  16 bits
    ar$hln (hardware address length) - 8 bits
    ar$pln (protocol address length) - 8 bits
    ar$op  (opcode) - 16 bits
    ar$sha (source hardware address) - n bytes,
                                     where n is from the ar$hln field.
    ar$spa (source protocol address) - m bytes,
                                     where m is from the ar$pln field.
    ar$tha (target hardware address) - n bytes
    ar$tpa (target protocol address) - m bytes
 ar$hrd, ar$pro, ar$hln and ar$pln are the same as in regular ARP
 (see [1]).

Finlayson, Mann, Mogul, Theimer [Page 2]

RFC 903 June 1984

 Suppose, for example, that 'hardware' addresses are 48-bit Ethernet
 addresses, and 'protocol' addresses are 32-bit Internet Addresses.
 That is, we wish to determine Internet Addresses corresponding to
 known Ethernet addresses.  Then, in each RARP packet, ar$hrd = 1
 (Ethernet), ar$pro = 2048 decimal (the Ethertype of IP packets),
 ar$hln = 6, and ar$pln = 4.
 There are two opcodes: 3 ('request reverse') and 4 ('reply reverse').
 An opcode of 1 or 2 has the same meaning as in [1]; packets with such
 opcodes may be passed on to regular ARP code.  A packet with any
 other opcode is undefined.  As in ARP, there are no "not found" or
 "error" packets, since many RARP servers are free to respond to a
 request.  The sender of a RARP request packet should timeout if it
 does not receive a reply for this request within a reasonable amount
 of time.
 The ar$sha, ar$spa, $ar$tha, and ar$tpa fields of the RARP packet are
 interpreted as follows:
 When the opcode is 3 ('request reverse'):
    ar$sha is the hardware address of the sender of the packet.
    ar$spa is undefined.
    ar$tha is the 'target' hardware address.
       In the case where the sender wishes to determine his own
       protocol address, this, like ar$sha, will be the hardware
       address of the sender.
    ar$tpa is undefined.
 When the opcode is 4 ('reply reverse'):
    ar$sha is the hardware address of the responder (the sender of the
    reply packet).
    ar$spa is the protocol address of the responder (see the note
    below).
    ar$tha is the hardware address of the target, and should be the
    same as that which was given in the request.
    ar$tpa is the protocol address of the target, that is, the desired
    address.
 Note that the requirement that ar$spa in opcode 4 packets be filled

Finlayson, Mann, Mogul, Theimer [Page 3]

RFC 903 June 1984

 in with the responder's protocol is purely for convenience.  For
 instance, if a system were to use both ARP and RARP, then the
 inclusion of the valid protocol-hardware address pair (ar$spa,
 ar$sha) may eliminate the need for a subsequent ARP request.

IV. References

 [1] Plummer, D., "An Ethernet Address Resolution Protocol",  RFC 826,
 MIT-LCS, November 1982.

Appendix A. Two Example Implementations for 4.2BSD Unix

 The following implementation sketches outline two different
 approaches to implementing a RARP server under 4.2BSD.
 A. Provide access to data-link level packets outside the kernel.  The
 RARP server is implemented completely outside the kernel and
 interacts with the kernel only to receive and send RARP packets.  The
 kernel has to be modified to provide the appropriate access for these
 packets; currently the 4.2 kernel allows access only to IP packets.
 One existing mechanism that provides this capability is the CMU
 "packet-filter" pseudo driver.  This has been used successfully at
 CMU and Stanford to implement similar sorts of "user-level" network
 servers.
 B. Maintain a cache of database entries inside the kernel.  The full
 RARP server database is maintained outside the kernel by a user
 process.  The RARP server itself is implemented directly in the
 kernel and employs a small cache of database entries for its
 responses.  This cache could be the same as is used for forward ARP.
 The cache gets filled from the actual RARP database by means of two
 new ioctls.  (These are like SIOCIFADDR, in that they are not really
 associated with a specific socket.)  One means: "sleep until there is
 a translation to be done, then pass the request out to the user
 process"; the other means: "enter this translation into the kernel
 table".  Thus, when the kernel can't find an entry in the cache, it
 puts the request on a (global) queue and then does a wakeup().  The
 implementation of the first ioctl is to sleep() and then pull the
 first item off of this queue and return it to the user process.
 Since the kernel can't wait around at interrupt level until the user
 process replies, it can either give up (and assume that the
 requesting host will retransmit the request packet after a second) or
 if the second ioctl passes a copy of the request back into the
 kernel, formulate and send a response at that time.

Finlayson, Mann, Mogul, Theimer [Page 4]

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